A conventional electric oil pump is provided in parallel to a mechanical oil pump driven by an internal combustion engine as a power source of a vehicle to thereby supply oil to a transmission of the vehicle. The electric oil pump supplies lubricating/cooling oil to at least a part (clutch) of the transmission as needed in response to an operation request.
In some cases, however, when the electric oil pump configured to operate in response to the operation request is stopped, oil retains in an oil pipe controlled by the electric oil pump, whereby its temperature differs from oil in an oil pan that frequently flows by means of the mechanical oil pump. The oil retaining in the oil pipe controlled by the electric oil pump tends to lower the temperature and increase the viscosity. Thus, when operating the electric oil pump in this state, the motor might suffer from an excessive resistance and fail to respond to the operation request.
As a countermeasure against the above problem, the following technique has been known (see Patent Document 1). That is, in order to prepare for the operation in advance of the request to operate the electric oil pump, the electric oil pump is rotated at a target rotational speed set to a second preoperation rotational speed. At least when the actual rotational speed matches a predetermined rotational speed not higher than the second preoperation rotational speed, it is determined that the preoperation is completed. Under at least one condition that it is not yet determined that the preoperation is completed, the electric oil pump is rotated at the target rotational speed set to a first preoperation rotational speed lower than the second preoperation rotational speed and the predetermined rotational speed.